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Lightness constancy refers to our ability to perceive the relative reflectance of objects despite changes in illumination. The easiest way to think of lightness constancy is to think of it along the continuum from black to gray to white. These achromatic lumincances simply refer to the amount of white light an object reflects. A black object absorbs most light-whereas a white object reflects most light, with gray objects being in between. Lightness constancy refers to the observation that we continue to see an object in terms of the proportion of light it reflects rather than the total amount of light it reflects. That is, a gray object will be seen as gray across wide changes in illumination. A white object remains white in a dim room, while a black object remains black in a well-lit room. In this sense, lightness constancy serves a similar function as color constancy in that it allows us to see properties of objects as being the same under different conditions of lighting (Adelson, 1993).

Consider an object that reflects 25% of the light that hits its surface. This object will be seen as a rather dark gray. If we leave it in a dim room that receives only 100 units of light, it will reflect 25 units of light. However, if we place it in a room that is better lit, it will still reflect the same 25%. If there are now 1,000 units of light, it will reflect 250 units of light. But we still see it as approximately the same gray, despite the fact that the object is reflecting much more light. Similarly, an object that reflects 75% of ambient light will be seen as a light gray in the dim room, even though it reflects less total light than it does in the bright room. Thus, lightness constancy is the principle that we respond to the proportion of light reflected by an object rather than the total light reflected by an object.

In this activity, you can experience lightness constancy via a classic illusion, the Cornsweet Illusion (Cornsweet, 1970) as modified by Purves, Shimpi, and Lotto (1999) to show how we use expectations about illumination to alter our perceptions of brightness of surfaces and thus help create lightness constancy. You will be able to adjust the intensity of one surface and adjust the surroundings and even the apparent tilt of squares to see how these variables alter our perception of the brightness of a surface.

Cornsweet T.N. (1970). Visual perception. New York: Academic.
Purves, D., Shimpi, A., & Beau Lotto (1999). An Emprical Explanation of the Cornsweet Effect. The Journal of Neuroscience, 19(19), 8542-8551.


Full Screen Mode

To see the illustration in full screen, which is recommended, press the Full Screen button, which appears at the top of the page.

Illustration Tab


On the Illustration tab, you can adjust these parameters:

Adjust Top Lightness: adjust how bright the top area is. Try to match it to the bottom area.
Clear Background and Shadows: add or remove the background and the shadows around the shapes. This is useful to check if you have made a match when adjusting the lightness.
Adjust Shadows: change the amount of shadow surrounding the shapes. If there is no shadow, how bright do the two areas appear relative to each other?
Tilt Squares: have the squares tilted or flat. If the two squares appear flat on the screen, having the same degree of light, does the illusion remain as strong?
Sky Color: change the color around the top shape. In particular, focus on the intensity (luminance) of the area.
Ground Color: change the color around the lower shape. In particular, focus on the intensity (luminance) of the area.


Pressing this button restores the settings to their default values and allows you to adjust speed and relative size. It also resets the counter before you can indicate if you have the objects arriving at the same time.